1. Field of the Invention
The present invention relates to an exposure apparatus used for transferring the image of a circuit pattern formed in a photo mask onto a substrate (or wafer) during the lithographic operation in the manufacture of semiconductor devices and a photo mask (or a reticle) used as a transferring negative used in the course of such operation.
2. Description of the Prior Art
Conventional exposure apparatus generally used during the lithographic operation in the manufacture of semiconductor devices is constructed for example as schematically shown in FIG. 8 of the accompanying drawings.
In the Figure, a photo mask 21 is held horizontally by a mask stage 25 so as to be perpendicular to the optical axis of an illumination optical system 24 and it is illuminated by the transmission of an exposure light of a given wavelength radiated from the illumination optical system 24. The photo mask 21, which has heretofore been used for general purposes, is so constructed that a light shielding pattern made of such metal as chromium is formed on a transparent substrate and the diffracted beams corresponding to the pattern configuration are produced as the result of the illumination by transmission. The diffracted beams are condensed on an imaging plane 23 by a projection optical system 22 so that an image of the pattern contained in the photo mask 21 is transferred onto the surface of a wafer held to conform with the imaging plane 23 by a wafer stage 26. At this time, the beams condensed on the imaging plane 23 represents a condition having no polarization characteristic, i.e., the average condition of a TE polarized light and a TM polarized light which will be described latter.
Referring now to FIG. 9, there is schematically illustrated the cross-sectional construction of a conventional photo mask formed with a line-and-space pattern composed of alternately repeated light transmitting portions (substrate bear surface portions) 14 and light shielding portions 12. Also, FIG. 10a shows schematically the planar construction of the conventional photo mask, and FIG. 10b shows an amplitude-transmittance distribution of the conventional photo mask.
As shown in these Figures, the amplitude-transmittance is low (=0) in the light shielding portions 12 of the line-and-space pattern and it is high in the light transmitting portions 14, thereby forming a fundamental period P.sub.1 of the pattern by the adjoining pair of the light shielding portion 12 and the light transmitting portion 14 as shown in FIG. 10b.
Even if the exposure is effected by use of the conventional photo mask of the type described above, however, there is the disadvantage of failing to meet the demand for finer circuit patterns due to the increase in the level of integration for semiconductor devices. Thus, there is the need for the development of a technique capable of forming a high-contrast image of a fine pattern.
Under these circumstances, as a means of enhancing the contrast of a pattern image, various phase shifting methods have recently been proposed in which projection exposure is effected by the use of a phase-shifted mask provided with phase shifting portions for varying the phase of transmitting beams at given locations of the light transmitting portions of the photo mask. For example, Japanese Patent Publication No. SHO 62-50811 discloses a technique relating to phase-shifted masks of a spatial frequency modulation type, etc. In accordance with the phase shifting method, a pattern image is formed by utilizing the phase information of light in addition to the amplitude information of light and therefore it is expected that some improvement of the imaging characteristic can be realized as compared with the method employing a photo mask composed of only light transmitting portions and light shielding portions.
Thus, the phase shifting method disclosed in the previously mentioned Japanese Patent Publication will now be described briefly with reference to FIGS. 11, 12a and 12b.
FIG. 11 shows schematically the cross-sectional construction of a conventional phase-shifted mask of the spatial frequency modulation type, FIG. 12a shows schematically its planar construction and FIG. 12b shows its amplitude-transmittance distribution. As will be seen from FIGS. 11 and 12a, in the line-and-space pattern the alternate light transmitting portions 14b are each provided with a phase shifter 15 so that the light beams transmitted through the light transmitting portions 14a and 14b adjoining each light shielding portion 12 on its sides have a phase difference of .pi. radians from each other. Also, as shown in FIG. 12b, the beam transmitted through the light transmitting portion 14b with the phase shifter 15 and the beam transmitted through the light transmitting portion 14a without the phase shifter 15 have amplitudes which are opposite in sign to each other and thus a fundamental period P.sub.2 of the pattern is formed by the light transmitting portion 14a (without the phase shifter), the light shielding portion 12, the light transmitting portion 14b (with the phase shifter) and the light shielding portion 12.
However, while, in the case of phase-shifted masks of the type developed recently, the contrast of a pattern image is enhanced by adding the phase information of light to the amplitude information of light, these techniques are as a matter of course limited in imaging performance and are still inadequate in ensuring a satisfactory high-contrast image for any fine pattern.
Moreover, while it has been confirmed that the use of the phase-shifted mask of the above mentioned type has the effect of improving the imaging characteristic, the actual state is such that a sufficient resolving power capable of resolving the required fine pattern has not been obtained as yet.